FLUX TUBES WITH A THIN TRANSITION LAYER - SCATTERING AND ABSORPTION PROPERTIES

In this paper, we use the T-matrix formalism to discuss the scattering and absorption properties of isolated flux tubes. We give a general e xpression for the T-matrix of a 1D Bur tube in terms of the normal aco ustic impedances for the different components of the acoustic wavefiel d. This shows how the (leaky and non-leaky) eigenmodes are related to those frequencies at which the normal acoustic impedances for the scat tered and the transmitted wavefield are equal. We specify flux tubes w ith a uniform core, and a thin transition layer (TTL) where the equili brium quantities change continuously to their photospheric levels. In this layer, resonant absorption can convert some of the incident acous tic power into heat. Using the Sakurai, Goossens, and Hollweg (1991) p rescription to treat the resonance analytically in linear ideal MHD, w e derive the analytic expression for the T-matrix of a TTL flux tube. From this expression, we show how leaky eigenmodes associated with the kink or tube speed of the uniform core of the ?TL flux tube drastical ly influence the scattering and absorption properties. In particular, we point out that discrete leaky modes accumulating in the range of th e ideal slow continuum, within the range of the ideal Alfven continuum , can severely alter the absorption strengths. This is of immediate im portance for thin magnetic flux tubes that are hotter than their immed iate surroundings (magnetic elements and clusters of magnetic elements ).